The present invention is generally concerned with ophthalmic surgery, and more particularly relates to systems, methods and apparatus for tracking the position of a human eye. The present invention is particularly useful for tracking the position of the eye during surgical procedures, such as photorefractive keratectomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), or the like. In an exemplary embodiment, the present invention is incorporated into a laser ablation system which is capable of modifying the spatial and temporal distribution of laser energy directed at the cornea based on the eye's position during the laser ablation procedure.
In ophthalmic surgery, the ability to optically track or follow the movement of the patient's tissue is recognized as a highly desirable element in laser delivery systems designed to effect precision surgery in delicate ocular tissue. This tracking of the eye includes not only the voluntary movements which can be damped with specialized treatment, but also the involuntary movements which are more difficult to control on a living patient. According to Adler's Physiology of the Eye, even when the patient is holding "steady" fixation on a visual target, eye movement still occurs. Further, involuntary head motion may occur that causes further motion of the eye. Such motion may compromise the efficacy of certain ocular surgical procedures requiring great precision. This motion may occur even when total immobilization of the eye of the eye is attempted. Total immobilization of the eye is not fully effective in suppressing involuntary eye motion, is rather uncomfortable for the patient and may cause potentially sight threatening elevations in intraocular pressure. The implementation of automatic tracking of the eye would alleviate any need for such immobilization and offer a technique for more effectively accommodating all types of eye motion. Thus, augmenting surgery with a real time eye tracking system may improve upon the accuracy and speed with which surgical procedures could be performed, as well as enabling new procedures to be carried out for the first time.
Various techniques have been described for tracking eye movements. The following references disclose techniques for tracking eye movements and are herein incorporated by reference in their entirety: Rashbass, Journal of the Optical Society of America, Vol. 50, pp. 642-644, 1960; Crane and Steele, Applied Optics, Vol. 24, pp. 527, 1985; U.S. Pat. No. 3,804,496 to Crane et al.; U.S. Pat. No. 4,443,075 to Crane; U.S. Pat. No. 5,231,674 to Cleveland et al.; U.S. Pat. No. 5,471,542 to Ragland; U.S. Pat. No. 5,604,818 to Saitou et al.; U.S. Pat. No. 5,632,742 to Frey; U.S. Pat. No. 5,752,950 to Frey; PCT International Publication Number WO 94/18883 by Knopp et al.; and PCT International Publication Number WO 95/27453 by Hohla.
Many of the known tracking techniques fall into one of two distinct categories, optical point trackers and digital image trackers, the latter including numerous variations of pattern recognition and edge detection methods. Optical point trackers utilize reflected images from various layers of the eye. These trackers optically distinguish reflected light to form images such as the first, second, third and fourth Purkinje images. For example, a dual Purkinje image technique compares the displacement of two different-order Purkinje images over time, and uses a repositioning apparatus to adjust the isometric transformation corresponding to the motion. A similar application of dual Purkinje technique to stabilize a visual system was used in a fundus illumination and monitoring device. These and similar Purkinje image-based tracking methods purport to follow the movement of the anterior surface of the eye. While such techniques possess, in principle, sufficient speed to follow the displacement of Purkinje points, they include an implicit assumption that the eye moves as a rigid body. During surgery, however, the eye does not move as a rigid body. Thus, localization of the Purkinje points can be influenced by transient relative motions between the various optical elements of the eye, which leads to fictitious position information for identifying the surface of the cornea. In addition, such systems are rather complex and tend to exhibit large variability between individuals in their calibration setting, which requires continuous real-time adjustments of the amplitude of the controlling signals. Also, during surgery of the eye the optical quality of the eye is temporarily degraded. This temporary degradation of the optical quality distorts and blurs the Purkinje images. Therefore, these blurry images make an accurate determination of the position of the eye very difficult.
Another class of tracking methods involve, in one form or another, digital image processing techniques. These techniques include retinal image trackers, various pattern recognition algorithms and edge detection techniques. In these cases, very fast frame-rate CCD cameras, sophisticated processing algorithms, and high speed computer processing are required along with fast servo-controlled mirrors for closing the loop. These requirements are generally caused by the large amounts of digital data produced by images used for image processing and the computational requirements for processing images. With the frequency response limited in practice to about one tenth the update frequency, digital image comparisons are considered to be relatively slow. In the case of tracking eye motions, setting the sampling frequency to an order of magnitude higher than the highest frequency to be pursued translates into kHz rates, leaving less than one thousandth of a second for processing the signal information.
Several other practical difficulties plague most image processing techniques including the need for rather prominent and recognizable features, which are often not easily located in the eye's structures during surgery. Also, techniques predicated upon high speed image processing of video signals are often deficient due to unfavorable tradeoffs between field of view, spatial resolution and frequency response. Specifically, since the image processing algorithms are limited by the size and spacing of the view elements (pixels), the digital methods do not afford continuous resolution. Increasing the resolution exacts penalties in terms of the field of view. Yet, relatively large areas should be acquired. One approach is to increase the number of pixel elements in an image sensor. Unfortunately, increasing pixel resolution significantly increases the system cost and degrades the system frequency response because of increasing image data and computations. Alternatively, fast moving optical deflectors and associated control circuitry may be employed. Unfortunately, this additional instrumentation also increases system cost and degrades the system response time. Consequently, the system will have an undesirable combination of diminished resolution, decreased response time or increased cost.
A more promising technique for tracking eye movement takes advantage of the differences in the light scattering properties of the iris and sclera. In this technique, light is projected onto the cornea/sclera interface or limbus, and the scattered light from the limbus is detected by photodetectors to determine an edge or boundary of a portion of the sclera and cornea. With this technique, the iris beneath the cornea will absorb light passing through the cornea and make the cornea adjacent the sclera appear dark. The relative position of this boundary can then be monitored to track the position of the eye.
The prior art techniques for tracking a boundary such as the limbus lack the desired combination of accuracy, speed and affordability that would be desirable for use with laser eye surgery. One technique of tracking the boundary of the cornea and sclera has been to project a single spot onto a portion of the limbus and vary the position of the spot along a line such that the light reflected onto a detector remains constant. The position of the projected spot is then assumed to represent the position of the limbus. Unfortunately, measurements which are taken of a portion of an object such as a single spot projected onto the limbus do not accurately represent the position of the entire object. A further disadvantage of tracking a single spot is that the portion of the limbus that is tracked may not be clearly visible or may change during surgery.
A further technique for tracking the limbus has been to utilize position sensing detectors, and offset a mirror to aim the element at a new offset position of the eye. These additional electronics typically involved with these techniques can increase the cost of the system and can decrease the system response time. Various factors also limit the effectiveness of this approach, particularly its sensitivity to individual variability among eyes, such as variations in iris diameter and varying contrast between the iris and sclera. Further, systems using this approach will typically only sample a limited portion of the limbus, and this portion of the sampled boundary may be covered by tissue during surgery.
Another problem with existing eye tracking systems which measure the position of the limbus occurs when the limbus is covered by tissue during surgery. An example of a surgical procedure that covers a portion of the limbus is laser in situ keratomileusis (LASIK). During this procedure, the epithelium, Bowman's membrane, and a portion of the anterior stroma are partially incised from the stroma and folded back to expose the stroma to the laser. The partially removed corneal tissue is typically folded back away from the center of the cornea and laid over a portion of the limbus. The incised tissue covering the limbus, however, is extremely rough and a poor optical surface. Accordingly, systems that rely on light reflection or scattering from this region of the eye do not provide meaningful data. Further, the position of the flap may vary among surgeons. This variability of flap position can cause further problems with prior art eye trackers. For example, a surgeon may not be able to perform LASIK as desired because his or her preferred orientation of the flap of incised tissue may cover a portion of the eye used by the eye tracker.
Laser surgery systems that have been integrated with eye trackers in the past have used the eye tracker to provide a central reference.
The performance of these integrated surgical laser and eye tracking systems is often less than optimal when used with the LASIK surgical procedure. With the LASIK surgical procedure, the central features of the cornea and underlying tissues are not easily located because of the rough corneal surface produced by the incision. Further, the laser treatment may change the corneal tissue and make tracking more difficult by changing the visibility of a tracked feature.
Another limitation of the prior art eye trackers has been the algorithms employed for coupling the offsetting of the laser beam to match the eye motion. For example, some systems repeatedly adjust an aiming beam toward an intended target until the two positions are aligned. This repeated adjustment of the laser beam will delay the laser treatment. Laser treatment delays are undesirable because they can cause the drying of the eye and too much tissue to be removed from the dried eye.
What is needed therefore are improved methods and apparatus for tracking the eye. In particular, these methods and apparatus should be capable of accurately tracking eye movements in real time so that these movements can be compensated for during, for example, a laser ablation procedure. It would be particularly desirable if these methods and apparatus could be used during procedures in which a portion of the outer reflective surface of the eye (i.e., the epithelium, and/or the anterior corneal tissue) is variably removed, such as in LASIK procedures. Further, it would be desirable if these eye tracking techniques were optimally integrated with a surgical laser system.